In recent years, due to demand for energy savings and shorter warm-up time (i.e., the amount of time between when the image forming apparatus is turned on and when the fixing device is ready for the fixing operation) in a fixing device, a belt-type fixing method used in image forming apparatuses has attracted attention in which heat capacities can be set to small values. Also in recent years, as a heating method used in fixing devices, an electromagnetic induction heating method (IH) which can provide quick and high-efficiency heating has attracted attention. For saving energy in fixing color images, many products that combine the belt-type fixing method with the electromagnetic induction heating method have been commercially available. When the belt-type fixing method and the electromagnetic induction heating method are combined, a device (coil) that generates a magnetic flux for electromagnetic, induction is often provided outside a fixing belt (so-called external IH). Using this arrangement is advantageous in that a coil that generates a magnetic flux for electromagnetic induction can be easily laid out and cooled, and that the belt can be directly heated.
In the electromagnetic induction heating method described above, various techniques have been developed to prevent a heated member, such as a fixing belt, from overheating in a non-sheet-passing region, in accordance with the width of a sheet that passes through the fixing device (sheet passing width). In particular, a size switching technique in external IH is known. In this technique, a ferrite center core that forms a magnetic path around a coil is provided, and the center core is configured to be rotated by power from a drive unit. Thus, a selection can be made as to whether induction heating is to be applied to the heated member by a magnetic flux generated by the coil, or induction heating is to be suppressed by blocking or suppressing the magnetic flux. With this technique, the amount of heat generation in the non-sheet-passing region of the heated member can be set to a value different from that in the sheet passing region.
If the fixing device described above has a configuration in which a unit on the side of the coil and the center core (hereinafter referred to as an electromagnetic-induction heating unit) and a unit on the side of the heated member (hereinafter referred to as a toner fusing unit) are combined as an integral unit, the entire fixing device needs to be replaced in the event of a problem that requires replacement of only one component of the fixing device. Since even components that do not need to be replaced become subject to replacement, there is a concern that costs involved in replacement cannot be reduced.
Such a concern can be resolved if the electromagnetic-induction heating unit and the toner fusing unit are configured separately. For example, it is possible that the electromagnetic-induction heating unit is disposed such that it cannot be removed from the image forming apparatus main body, whereas the toner fusing unit is configured such that it can be attached to and removed from the electromagnetic-induction heating unit.
However, if the electromagnetic-induction heating unit and the toner fusing unit are configured to be separated from each other, the drive unit and the electromagnetic-induction heating unit of the fixing device may be easily damaged when the toner fusing unit is attached to the electromagnetic-induction heating unit.
Specifically, to make the axis of the center core parallel to that of the heat roller at the point when the toner fusing unit is attached to the electromagnetic-induction heating unit, it is necessary that the electromagnetic-induction heating unit be mounted on the image forming apparatus main body with some degree of freedom (i.e., such that the electromagnetic-induction heating unit is movable relative to the image forming apparatus main body) until the toner fusing unit is attached. In this case, if the drive unit is disposed in the image forming apparatus main body, there is a possibility that a connecting component of the drive unit may hit that of the toner fusing unit when the toner fusing unit is attached to the electromagnetic-induction heating unit. More specifically, since a speed reduction gear train of the drive unit and a driven gear of the toner fusing unit are brought into contact with each other in an unstable state, a problem such as improper gear engagement, possibly resulting in damage and/or loss of a gear tooth, may occur when the toner fusing unit is mounted. It may be possible to provide a positioning mechanism for positioning the speed reduction gear train and the driven gear. However, since this adds complexity to the existing mechanism, it is difficult to reduce the manufacturing cost and the size of the apparatus.
In solving such a problem, it is to be noted, that the drive unit typically may be disposed at a corner of the image forming apparatus main body, and that in such a case the drive unit cannot easily come into contact with a flow of air for cooling the interior of the image forming apparatus main body. This lack of cooling may have deleterious effects on performance or reliability because if components of the drive unit are heated to temperatures that exceed the upper temperature limits that the components can bear, these components cannot fully demonstrate their performance.